1. Field
The embodiments described herein relate to a semiconductor memory device and an operation method thereof.
2. Description of the Related Art
In recent years, resistive memory devices utilizing a variable resistor as a memory element are receiving attention as candidates to succeed flash memory. The resistive memory devices herein are assumed to include not only resistive RAM (ReRAM), in a narrow sense, that uses a transition metal oxide as a recording layer and stores its resistance states in a non-volatile manner, but also the likes of Phase Change RAM (PCRAM) that uses chalcogenide or the like as a recording layer to utilize the resistance information of crystalline states (conductors) and amorphous states (insulators).
Two kinds of operation modes in memory cells of resistive memory devices are known. In one kind, known as a bipolar type, a high-resistance state and a low-resistance state are set by switching a polarity of an applied voltage. In the other kind, known as a unipolar type, setting of the high-resistance state and the low-resistance state are made possible by controlling a voltage value and a voltage application time, without switching the polarity of the applied voltage.
The unipolar type is preferable for realizing a high-density memory cell array. This is because, in the case of the unipolar type, the cell array can be configured by overlapping a variable resistor and a rectifier such as a diode at intersections of bit lines and word lines, without using a transistor. Furthermore, arranging such memory cell arrays three-dimensionally in stacks enables a large capacity to be realized without causing an increase in cell array area.
In the case of unipolar type ReRAM, write of data to a memory cell is performed by applying a certain voltage to the variable resistor for a short time. This causes the variable resistor to change from the high-resistance state to the low-resistance state. This operation to change the variable resistor from the high-resistance state to the low-resistance state is hereinafter referred to as a setting operation. On the other hand, erase of data in the memory cell is performed by applying a certain voltage to the variable resistor for a long time, the certain voltage being lower than that applied during the setting operation, and the variable resistor being in the low-resistance state subsequent to the setting operation. This causes the variable resistor to change from the low-resistance state to the high-resistance state. This operation to change the variable resistor from the low-resistance state to the high-resistance state is hereinafter referred to as a resetting operation. The memory cell adopts, for example, the high-resistance state as a stable state (reset state), and, in the case of binary data storage, write of data is performed by the setting operation in which the reset state is changed to the low-resistance state.
A read operation of the memory cell is performed by applying a certain voltage to the variable resistor and using a differential amplifier to monitor a current flowing through the variable resistor. This read operation allows determination of whether the variable resistor is in the low-resistance state or the high-resistance state. It is required in this kind of semiconductor memory device that the read operation causes data to be read reliably from a selected memory cell, even if there are dispersions in characteristics of the variable resistors.